Realizing the potential of otoacoustic emissions (OAEs) as noninvasive probes of cochlear function requires understanding the physical and physiological mechanisms that generate and shape these sounds. Over the next five years we propose innovative modeling and measurements to address important issues of cochlear function while improving our understanding of OAE generation. Aim 1 explores cochlear nonlinearities using OAEs. Aim 1a studies the form of cochlear gain control using OAE measurements and models to probe the dynamics of compression and suppression using paired clicks. Aim 1b studies the action of suppressor tones on OAE generation by testing their ability to map out the distribution of OAE generators in models where the distribution is known. Aim 2 studies how OAEs depend on mechanisms of cochlear amplification. Aim 2a explores OAE generation in response-matched cochlear models that employ push-pull amplification to test arguments that push- pull models cannot produce realistic OAEs. Aim 2b studies spontaneous OAEs (SOAEs) in models of the lizard cochlea-a species where the biophysics of amplification is fundamentally different from the mammal-to test the hypothesis that lizard SOAEs nevertheless arise through mechanisms analogous to those in mammals. Aim 3 probes cochlear apical/basal differences using OAEs. Aim 3a pursues the emerging idea that the base and apex are very different by building on our discovery of an otoacoustic/mechanical transition near the midpoint of the cochlea. Models derived from data will test the hypothesis that the OAE transition results from wave reflection at the mechanical seam. Aim 3b uses measurements and models to characterize apical mechanics and the OAE transition, testing hypotheses that relate the transition to the breaking of scaling symmetry. Completion of these Aims will significantly enhance our understanding of OAE generation and its relationship to cochlear mechanics across species and along the cochlea. The Aims are also directly relevant to improving the power of OAE-based diagnostics and other technological applications, such as hearing aids and preprocessors for speech-recognition devices that benefit from knowledge of cochlear amplification, nonlinearity, and signal processing.